A simplified methodology is rigorously studied in this article to analyze the modal properties of base-isolated high-rise structures with dynamic soil-structure interaction being considered. The proposed methodology is developed based on a more reasonable 2-degree-of-freedom model and the existing simplified methodology which is only applicable for nonisolated structures. The base-isolated structure model with 2 degrees of freedom is supported by swaying and rocking springs and by the corresponding dashpots. Rigorous mathematical derivation is performed, and closed-form formulas of natural periods, modes, and modal damping ratios are derived. Furthermore, the overall accuracy of the proposed methodology was checked against the results of the rigorously derived complex eigenvalue approach proposed by Constantinou and Kneifati. A parametric study is also conducted on the soil-structure interaction effects of baseisolated structures, which indicates that tall and slender structures with stiff isolation systems are more affected by soilstructure interaction effects in comparison to flexible superstructures. The proposed method provides a feasible way to evaluate the soil-structure interaction effects of base-isolated structures efficiently during the schematic design phase.